Veteran horse trainer Gai Waterhouse has been fined $5500 after she was found guilty of breaching two horse racing rules in her high-profile stoush with businessman John Singleton over the poor performance of his prized mare More Joyous.

Racing NSW ruled on Monday that Mrs Waterhouse had failed to report a condition that could affect the running of a horse and kept incomplete ­treatment records for the $4.5 million mare, which had a sore neck when it ran second last in the All Aged Stakes at Randwick on April 27.